Nonvolatile semiconductor memory device and method of rewriting data thereof

ABSTRACT

The nonvolatile semiconductor memory device of the present invention includes a memory cell array wherein data is stored in a nonvolatile state based on a difference in memory information between two memory cells comprising a memory cell pair, and a writing controller for writing data to the memory cell array. The writing controller is capable of individually setting memory information of each of the memory cells in the memory cell array.

This application is a divisional application of Ser. No. 11/087,734,filed Mar. 24, 2005, entitled “Nonvolatile Semiconductor Memory Deviceand Method of Rewriting Data Thereof”, now U.S. Patent No. 7,209,391which claims priority of Japanese Application No. 2004-097746, filedMar. 30, 2004, the contents of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nonvolatile semiconductor memorydevice including a plurality of memory cells.

2. Description of the Background Art

Japanese Patent Application Laid-Open No. 2002-025286 disclosesconventional nonvolatile semiconductor memory devices such as EPROM(Erasable Programmable ROM) and EEPROM (Electrically ErasableProgrammable ROM). According to a nonvolatile semiconductor memorydevice disclosed in Japanese Patent Application Laid-Open No.2002-025286, the difference between a signal from a certain memory celland a predetermined reference signal is amplified by a differentialsense amplifier, and the resultant output signal is considered as datawritten in the memory cell. Japanese Patent Application Laid-Open No.2001-043691 also discloses a nonvolatile semiconductor memory device.

As described above, the differential sense amplifier employed in aconventional nonvolatile semiconductor memory device compares a signalfrom a memory cell with a predetermined reference signal. When a noiseand the like generate under this configuration, it is difficult tosecure a signal difference with enough amplitude at the input of thedifferential sense amplifier, thereby causing the differential senseamplifier to malfunction at times. As a result, reliability of thenonvolatile semiconductor memory device cannot be secured to asatisfactory degree in such a case.

SUMMARY OF THE INVENTION

An object of the present invention is to provide technology forimproving the reliability of a nonvolatile semiconductor memory device.

The nonvolatile semiconductor memory device of the present inventionincludes a memory cell array having a plurality of memory cells, and awriting controller for writing data into the memory cell array. Thememory cell array stores data in a nonvolatile state with respect toeach memory cell pair composed of two memory cells. The data storing isexecuted based on a difference in memory information between the twomemory cells included in one memory cell pair. The writing controller iscapable of setting the memory information individually for each of thememory cells of the memory cell array.

Since the memory information of each of the memory cells of the memorycell array can be set individually, upon writing data into the memorycell array, it is possible to set a large difference in the memoryinformation between two memory cells included in one memory cell pair.This prevents a malfunction of a circuit for detecting a difference insuch memory information, and as a result, the nonvolatile semiconductormemory device of the present invention will have an improvedreliability.

The present invention is also intended for a method of rewriting data ofthe nonvolatile semiconductor memory device of the present invention.The nonvolatile semiconductor memory device includes a memory cell arrayhaving a plurality of memory cells, and a writing controller for writingdata into the memory cell array. The method of rewriting data has steps(a) and (b). The memory cell array stores data in a nonvolatile statewith respect to each memory cell pair composed of two memory cells. Thedata storing is executed based on a difference in memory informationbetween the two memory cells. Each of the memory cells of the memorycell array is a memory cell transistor having a control gate and afloating gate. The writing controller is capable of setting thethreshold voltage of each of the memory cells of the memory cell arrayto three or more levels. The writing controller sets the memoryinformation of each of the memory cells individually by setting athreshold voltage of each of the memory cells of the memory cell array.The nonvolatile semiconductor memory device further includes a pluralityof differential sense amplifiers for amplifying the output difference oftwo memory cells included in the corresponding memory cell pair foroutputting. The step (a) is to write data into said memory cell pair,and the step (b) is to rewrite the data written in the step (a). In thestep (a), the threshold voltage of one memory cell included in thememory cell pair is set to a higher level than that of the thresholdvoltage of the other memory cell in the memory cell pair, while thethreshold voltage of the other memory cell is set to a level chosen fromthree or more levels except the greatest one. In the step (b), thethreshold voltage of one memory cell is increased to a level greaterthan the threshold voltage of the other memory cell.

Since the data of the memory cell pair can be rewritten withoutinvolving a data erasing operation, the rewriting of data can beperformed easily.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the structure of a nonvolatilesemiconductor memory device according to a preferred embodiment of thepresent invention.

FIG. 2 is a block diagram illustrating the structure of a nonvolatilesemiconductor memory device according to the preferred embodiment of thepresent invention.

FIG. 3 is a circuit diagram illustrating the structure of a differentialsense amplifier according to the preferred embodiment of the presentinvention.

FIG. 4 shows signal waveforms generated in a differential senseamplifier according to the preferred embodiment of the presentinvention.

FIG. 5 is a circuit diagram illustrating the structure of a majoritydecision element according to the preferred embodiment of the presentinvention.

FIG. 6 is a truth table listing input value combinations and theircorresponding output values regarding a majority decision elementaccording to the preferred embodiment of the present invention.

FIG. 7 is a block diagram illustrating the structure of a start-updetector according to the preferred embodiment of the present invention.

FIG. 8 is a circuit diagram illustrating the structure of a firstvoltage detector according to the preferred embodiment of the presentinvention.

FIG. 9 is a circuit diagram illustrating the structure of a secondvoltage detector according to the preferred embodiment of the presentinvention.

FIG. 10 shows the voltage waveforms of a supply voltage and aninternally boosted voltage BOOST at power-on.

FIG. 11 is a flow chart illustrating operations of a nonvolatilesemiconductor memory device according to the preferred embodiment of thepresent invention.

FIGS. 12A and 12B show values (threshold voltages) set for a memory cellaccording to the preferred embodiment of the present invention.

FIGS. 13A and 13B show values (threshold voltages) set for a memory cellaccording to the preferred embodiment of the present invention.

FIGS. 14A and 14B show values (threshold voltages) set for a memory cellaccording to the preferred embodiment of the present invention.

FIGS. 15A and 15B show values (threshold voltages) set for a memory cellaccording to the preferred embodiment of the present invention.

FIG. 16 is a flow chart illustrating operations of a nonvolatilesemiconductor memory device according to the preferred embodiment of thepresent invention when the threshold voltage setting for each memorycell is limited to two kinds only.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram illustrating the structure of a nonvolatilesemiconductor memory device (hereinafter simply referred to as“semiconductor memory device”) according to a preferred embodiment ofthe present invention. The semiconductor memory device according to thepresent preferred embodiment is, for example, an EPROM on which data canbe erased using ultraviolet light, and is incorporated into asemiconductor integrated circuit comprising elements such as a CPU and aDRAM. Further, information to be stored in the semiconductor memorydevice according to the present preferred embodiment includes, forexample, chip information containing information for adjusting aninternal voltage in the semiconductor integrated circuit and a lotnumber and the like, and replacement information for repairing a defectmemory cell in a DRAM by using a redundant cell.

As shown in FIG. 1, the present semiconductor memory device includes amemory cell array 6 for storing data in a nonvolatile state, a writingcontroller 1 for writing data into the memory cell array 6, a readingcontroller 2 for reading data from the memory cell array 6, an addressdecoder 3, and a gate signal switching circuit 13.

Upon reading data from the memory cell array 6, the reading controller 2sends to the address decoder 3 an address signal ADR generated by anaddress counter 2 a provided inside the controller. Upon receiving theaddress signal ADR, the address decoder 3 decodes the address signal ADRand then sends a word line activation signal for reading RAWL to thegate signal switching circuit 13.

A write command WRCOM from a CPU and the like (not shown) prompts thewriting controller 1 to write data into the memory cell array 6. Uponwriting data, the writing controller 1 sends a word line activationsignal for writing WAWL to the gate signal switching circuit 13.

The gate signal switching circuit 13 sends to a word line WL either oneof the word line activation signal for reading RAWL or word lineactivation signal for writing WAWL based on the value of a write controlsignal WRC provided by the writing controller 1. More specifically, whenthe write control signal WRC is “0”, the gate signal switching circuit13 outputs the word line activation signal for reading RAWL, and whenthe write control signal WRC is “1”, it outputs the word line activationsignal for writing WAWL. As a result, a gate voltage for reading Vg1 isapplied to the word line WL of the memory cell array 6 upon reading, anda gate voltage for writing Vg2 is applied to the word line WL of thememory cell array 6 upon writing.

Further, the semiconductor memory device according to the presentpreferred embodiment includes a bit line selection circuit for writing4, a gate circuit for writing 5, a gate circuit for reading 7, anamplification circuit 8, a decision circuit 9; and a latch circuit 10.The operations of the bit line selection circuit for writing 4 and thegate circuit for writing 5 are controlled by the writing controller 1.The operations of the gate circuit for reading 7 and the amplificationcircuit 8 are controlled by the reading controller 2. Furthermore, thesemiconductor memory device according to the present preferredembodiment includes a boost circuit 11 for boosting a supply voltage Vddto be output as an internally boosted voltage BOOST, and a start-updetector 12 for detecting a start-up of the present semiconductor memorydevice based on the supply voltage Vdd and the internally boostedvoltage BOOST.

FIG. 2 is a block diagram illustrating the structures of the bit lineselection circuit for writing 4, the gate circuit for writing 5, thememory cell array 6, the gate circuit for reading 7, the amplificationcircuit 8, the decision circuit 9 and the latch circuit 10. As shown inFIG. 2, the memory cell array 6 comprises (m×n) pieces of memory cellsMC (where n≧2 and m≧2), m number of word lines WL extending row-wise, nnumber of bit lines BL extending column-wise, and m number of sourcelines SL extending row-wise. Each of the source lines SL is providedwith a ground potential.

A plurality of memory cells MC provided in a matrix are memory celltransistors each comprising a control gate and a floating gate. Each ofthe word lines WL is connected to control gates of n number of memorycells MC lined row-wise. Each of the bit lines BL is connected to drainsof m number of memory cells MC lined column-wise. Each of the sourcelines SL is connected to sources of n number of memory cells MC linedrow-wise.

The memory cell array 6 according to the present preferred embodimenthas a structure wherein two memory cells MC adjacent row-wise comprise amemory cell pair MCP serving as a unit of data storage. Under thisconfiguration, (n/2) pieces of memory cell pairs MCP are connected toeach of the word lines WL, and (m×n/2) pieces of memory cell pairs MCPare included in the memory cell array 6. The memory cell array 6 storesdata with respect to each memory cell pair MCP as a unit of datastorage, based on a difference in memory information between two memorycells MC included in one memory cell pair MCP. That is, in the presentpreferred embodiment, a difference in memory information between thememory cells MC of a certain memory cell pair MCP causes data to bewritten into that particular memory cell pair MCP. This data writing isdescribed in detail in the following.

In the present preferred embodiment, a memory transistor including acontrol gate and a floating gate is employed as the memory cell MC. Assuch, the setting of memory information of the memory cell MC isdetermined by setting the threshold voltage of the memory cell MC.Therefore, if two memory cells MC have a threshold voltage differentfrom each other, the memory cells have memory information different fromeach other. The memory cell array 6 of the present preferred embodimentstores data based on a difference in the threshold voltage between thememory cells MC included in one memory cell pair MCP. In other words, adifference in the threshold voltage between two memory cells MC in acertain memory cell pair MCP causes data to be written into thatparticular memory cell pair MCP.

In the present preferred embodiment, in order to write data “1” into acertain memory cell pair MCP composed of two memory cells MC, thethreshold voltage of one memory cell MC is set lower than the thresholdvoltage of the other memory cell MC. In contrast, in order to write data“0” into a certain memory cell pair MCP composed of two memory cells MC,the threshold voltage of one memory cell MC is set higher than thethreshold voltage of the other memory cell MC. Hereinafter, the memorycell with a lower threshold voltage when data “1” is written into thememory cell pair MCP is referred to as a “true-side memory cell MC”, andthe remaining memory cell MC is referred to as a “bar-side memory cellMC”.

For example, assume a situation where a certain memory cell MC has athreshold voltage of 1.5 V provided that hot electrons are not injectedinto that particular memory cell. In this case, hot electrons areinjected only into the floating gate of the bar-side memory cell MC ofthe memory cell pair MCP, thereby to set the threshold voltage thereofat 6 V. As a result, the threshold voltage (1.5 V) of the true-sidememory cell MC of the memory cell pair MCP becomes lower than thethreshold voltage (6 V) of the bar-side memory cell MC, and data “1” iswritten into the memory cell pair MCP.

In contrast, in order to write data “0”, hot electrons are injected onlyinto the floating gate of the true-side memory cell MC of the memorycell pair MCP, thereby to set the threshold voltage thereof at 6 V. Thiscauses the threshold voltage (6 V) of the true-side memory cell MC to behigher than the threshold voltage (1.5 V) of the bar-side memory cellMC, and data “0” is written into the memory cell pair MCP.

In this manner, in the present preferred embodiment, with regard to thememory cell array 6, data is written with respect to each memory cellpair MCP. Further, according to data value to be written, relative sizeof threshold voltage between two memory cells MC included in one memorycell pair MCP is manipulated. Such a writing control is carried out bythe writing controller 1, description thereof is to be given later.

The gate circuit for reading 7 comprises n number of NMOS transistors 7a provided in a one-to-one relationship to n number of bit lines BL, asshown in FIG. 2. A source of each of the NMOS transistors 7 a isconnected to one end of its corresponding bit line BL, and gates of theNMOS transistors 7 a are connected to each other. The gate of the NMOStransistor 7 a is provided with a read control signal RDC from thereading controller 2. The read control signal RDC turns n number of NMOStransistors 7 a simultaneously on when data is read from the memory cellarray 6.

The amplification circuit 8 comprises x number of differential senseamplifier groups SAG (x≧2). Each of the differential sense amplifiergroups SAG comprises three differential sense amplifiers SA0 to SA2. Thedecision circuit 9 comprises x number of majority decision elements JD,which are provided in a one-to-one relationship to x number ofdifferential sense amplifier groups SAG.

Each of the differential sense amplifier groups SAG has a configurationwhere two inputs of the differential sense amplifier SA0 arerespectively connected to drains of two NMOS transistors 7 a which arerespectively connected to two adjacent bit lines BL. In the same manner,two inputs of the differential sense amplifiers SA1, SA2 arerespectively connected to drains of two NMOS transistors 7 a which arerespectively connected to two adjacent bit lines BL.

Accordingly, while the NMOS transistors 7 a are in an ON state, twoinputs of the respective differential sense amplifiers SA0 to SA2 areelectrically connected to two bit lines BL. Each of the differentialsense amplifiers SA0 to SA2 amplifies the difference in output betweenthe true-side memory cell MC and the bar-side memory cell MC included inone memory cell pair MCP that is electrically connected to two bit linesBL. The amplified results in the differential sense amplifiers SA0 toSA2 are then respectively output as data SAO0 to SAO2.

The majority decision element JD obtains a majority logic of the dataSAO 0 to SAO2 output from their corresponding differential senseamplifiers SA0 to SA2 included in the differential sense amplifier groupSAG. In the present preferred embodiment, the same data is written intothree memory cell pairs MCP which are electrically connected to onedifferential sense amplifier group SAG. Based on the majority logicobtained from the data SAO0 to SAO2, the majority decision element JDaccording to the present preferred embodiment decides the data writteninto the three memory cell pairs MCP. The decision result then is outputas data PD.

In the latch circuit 10, x number of data PD output from the majoritydecision element JD are latched, then output as x bits of read data RDOto RDx−1. These read data RD0 to RDx−1 are read data output to theoutside of the present semiconductor memory device. Receiving the readdata RD0 to RDx−1, a CPU and the like provided adjacent to the presentsemiconductor memory device recognize information written in the presentsemiconductor memory device.

As stated above, in the present preferred embodiment, each of thedifferential sense amplifier group SAG is connected to six bit lines BLso that the relationship between the number of the bit line BL (n) andthe number of the majority decision element JD (x) is expressed by thefollowing: n=x×6.

The gate circuit for writing 5 comprises n number of NMOS transistors 5a provided in a one-to-one relationship to n number of bit lines BL, asshown in FIG. 2. A source of each of the NMOS transistors 5 a isconnected to the other end of its corresponding bit line BL, and gatesof the NMOS transistors 5 a are connected to each other. The gate of theNMOS transistor 5 a is provided with the write control signal WRC fromthe writing controller 1. The write control signal WRC turns n number ofNMOS transistors 5 a simultaneously on when data is written into thememory cell array 6.

The bit line selection circuit for writing 4 comprises n number of ANDcircuits 4 a and n number of NMOS transistors 4 b. One of the ANDcircuits 4 a and one of the NMOS transistors 4 b make a pair. Thecircuit comprising a pair of one AND circuit 4 a and one NMOS transistor4 b is provided in a one-to-one relationship to each of the bit linesBL.

One input of each of the AND circuits 4 a is provided with theabove-described write control signal WRC. N number of the other inputsof the AND circuits 4 a are respectively provided with n bits of bitline selection signals D0 to Dn−1 output from the writing controller 1.An output of the AND circuits 4 a is connected to the gate of respectivecounterparting NMOS transistors 4 b.

A source of each of the NMOS transistors 4 b is connected to a drain ofthe NMOS transistor 5 a connected to the corresponding bit line BL. Adrain voltage for writing VPD is applied to a drain of each of the NMOStransistors 4 b.

Next, detailed description is given regarding the circuit structure ofthe differential sense amplifier SA0. It is noted that the differentialsense amplifiers SA1 and SA2 have the same circuit structure as thedifferential sense amplifier SA0 and thus description of these twodifferential sense amplifiers SA1 and SA2 are omitted.

FIG. 3 is a circuit diagram illustrating the structure of thedifferential sense amplifier SA0. In FIG. 3, for convenience indescription, a bit line to which the true-side memory cell MC of thememory cell pair MCP is shown as “bit line BLa”, and a bit line to whichthe bar-side memory cell MC of the memory cell pair MCP is shown as “bitline BLb”.

As shown in FIG. 3, the supply voltage Vdd is applied to thedifferential sense amplifier SA0 as its supply voltage. The differentialsense amplifier SA0 comprises an AND circuit 8 a, PMOS transistors 8 bto 8 f and NMOS transistors 8 g to 8 k. One input of the AND circuit 8 ais provided with a sense amplifier enable signal SAE output from thereading controller 2, and the other input of the AND circuit 8 a isconnected to a sense amplifier bit line SABLa. The AND circuit 8 aperforms logical operation to obtain a logical product of the senseamplifier enable signal SAE and the signal value of the sense amplifierbit line SABLa, and outputs the result as data SAO0.

A drain of the PMOS transistors 8 b and 8 c is connected to the senseamplifier bit line SABLa and SABLb, respectively. The supply voltage Vddis applied to respective sources of the PMOS transistors 8 b and 8 c.Respective gates of the PMOS transistors 8 b and 8 c are supplied with acharge enable signal EQ output from the reading controller 2.

The supply voltage Vdd is applied to a source of the PMOS transistor 8d. A drain of the PMOS transistor 8 d is connected to respective sourcesof the PMOS transistors 8 e and 8 f. An inverted sense amplifier enablesignal /SAE output from the reading controller 2 is supplied to a gateof the PMOS transistor 8 d. The inverted sense amplifier enable signal/SAE is an inversion signal of the sense amplifier enable signal SAE.

Drains of the PMOS transistor 8 e and the NMOS transistor 8 g areconnected to each other, and also connected to the sense amplifier bitline SABLa. The drains of the PMOS transistor 8 e and the NMOStransistor 8 g are further connected to gate of the PMOS transistor 8 fand gate of the NMOS transistor 8 h. Drains of the PMOS transistor 8 fand the NMOS transistor 8 h are connected to each other, and alsoconnected to the sense amplifier bit line SABLb. The drains of the PMOStransistor 8 f and the NMOS transistor 8 h are further connected to gateof the PMOS transistor 8 e and gate of the NMOS transistor 8 g. Drain ofthe NMOS transistor 8 i is connected to respective sources of the NMOStransistors 8 g and 8 h. A ground potential is applied to a source ofthe NMOS transistor 8 i.

Drains of the NMOS transistors 8 j and 8 k are connected to the senseamplifier bit lines SABLa and SABLb, respectively. A source of the NMOStransistor 8 j is connected to a drain of the NMOS transistor 7 a whichis connected to the bit line BLa. A source of the NMOS transistor 8 k isconnected to a drain of the NMOS transistor 7 a which is connected tothe bit line BLb. Respective gates of the NMOS transistors 8 j and 8 kare provided with a sense amplifier input enable signal SAI output fromthe reading controller 2.

Next, description is given of operations of the differential senseamplifier SA0 according to the present preferred embodiment with such aconfiguration. In the following, description is given of the operationsof the differential sense amplifier SA0 when the true-side memory cellMC has a higher threshold voltage than the bar-side memory cell MC,where the true-side memory cell MC and the bar-side memory cell MC areincluded in the memory cell pair MCP which is connected to thedifferential sense amplifier SA0. It is noted that the differentialsense amplifiers SA1 and SA2 operate in the same manner as thedifferential sense amplifier SA0 and thus description of these twodifferential sense amplifiers SA1 and SA2 are omitted.

FIG. 4 shows respective signal waveforms generated in the differentialsense amplifier SA0. When one of the word lines WL is activated therebyturning the NMOS transistors 7 a on in the gate circuit for reading 7,the bit lines BLa and BLb and the source of the NMOS transistors 8 j and8 k are electrically connected, respectively. FIG. 4 shows a state inwhich the sense amplifier input enable signal SAI is at a Low level withboth the NMOS transistors 8 j and 8 k in an OFF state. In such a state,when the charge enable signal EQ enters a Low level, the PMOStransistors 8 b and 8 c enters an ON state, initiating the charging ofthe sense amplifier bit lines SABLa and SABLb and increasing thepotentials thereof to the level of the supply voltage Vdd.

Then, when both the charge enable signal EQ and the sense amplifierenable signal SAI enter a High level, the PMOS transistors 8 b and 8 cturn off and the NMOS transistors 8 j and 8 k turn on. As a result, thebit lines BLa and BLb are electrically connected to the sense amplifierbit lines SABLa and SABLb, respectively, causing a current to flowthrough the true-side memory cell MC and the bar-side memory cell MC andreducing the potentials of the sense amplifier bit lines SABLa andSABLb.

It is noted that a smaller current flows through the memory cell MC witha higher threshold voltage. Since the true-side memory cell MC has ahigher threshold voltage than the bar-side memory cell MC, the currentflowing through the true-side memory cell MC is smaller than the currentflowing through the bar-side memory cell MC. Accordingly, the senseamplifier bit line SABLa experiences a smaller voltage drop than thesense amplifier bit line SABLb does, and the sense amplifier bit lineSABLa exhibits a larger potential than the sense amplifier bit lineSABLb does. As a result, a minute potential difference is generatedbetween the sense amplifier bit line SABLa and the sense amplifier bitline SABLb.

With such a minute potential difference existing between the senseamplifier bit line SABLa and the sense amplifier bit line SABLb, whenthe sense amplifier input enable signal SAI and the inverted senseamplifier enable signal /SAE enter a Low level and the sense amplifierenable signal SAE enters a High level, the potential difference isamplified by a differential amplification circuit comprising the PMOStransistors 8 e and 8 f and the NMOS transistors 8 g and 8 h. Thisbrings the potential of the sense amplifier bit line SABLa up to thelevel close to the supply voltage Vdd and the potential of the senseamplifier bit line SABLb down to the level of a ground potential. As aresult, a High level input is supplied to both inputs of the AND circuit8 a, and the AND circuit 8 a generates “1” as its output data SAO0.

As described above, in the differential sense amplifier SA0, thedifference in output between the true-side memory cell MC and thebar-side memory cell MC, i.e. the difference in drain current betweenthe true-side memory cell MC and the bar-side memory cell MC, isconverted to a potential difference which corresponds to the magnitudeof the difference. Further, the potential difference is amplified. Thenas an amplified result based on output difference between the true-sidememory cell MC and the bar-side memory cell MC, data with a logicalvalue (“1” in the above example) contrary to the logical value (“0” inthe above example) written in the memory cell pair MCP is output fromthe differential sense amplifier SA0 as data SAO0.

Next, a detailed description is given of the majority decision elementJD. FIG. 5 is a circuit diagram illustrating the structure of themajority decision element JD. As shown in FIG. 5, the majority decisionelement JD comprises NOT circuits 9 a to 9 c, NAND circuits 9 d to 9 f,and a three-input OR circuit 9 g. The NOT circuits 9 a, 9 b and 9 cinvert the data SAO0, SAO1 and SAO2 output from the differential senseamplifiers SA0, SA1 and SA2 for outputting, respectively.

The NAND circuit 9 d carries out a negative logical product operation ofthe outputs from the NOT circuits 9 a and 9 b for outputting. The NANDcircuit 9 e carries out a negative logical product operation of theoutputs from the NOT circuits 9 b and 9 c for outputting. The NANDcircuit 9 f carries out a negative logical product operation of theoutputs from the NOT circuits 9 a and 9 c for outputting. The OR circuit9 g finds a logical sum of inverted signals of the outputs from the NANDcircuits 9 a to 9 c. Then, the obtained logical sum is output as dataPD.

FIG. 6 is a truth table listing input value combinations and theircorresponding output values regarding the majority decision element JD.As shown in FIG. 6, the majority decision element JD outputs an invertedsignal of the value that occurs most often among the values carried bythe data SAO0 to SAO2. That is, the majority decision element JD obtainsa majority logic of the data SAO0 to SAO2, then outputs the invertedsignal of the majority logic as the data PD.

In this manner, the majority decision element JD according to thepresent preferred embodiment obtains the majority logic of the data SAO0to SAO2, thereby to decide data written to three memory cell pairs MCPwhich are electrically connected to one differential sense amplifiergroup SAG. For instance, when the majority logic of the data SAO0 toSAO2 exhibits “0”, the majority decision element JD decides that thedata written to the memory cell pairs MCP are “0” and outputs “1” as thedecision result. Similarly, when the majority logic of the data SAO0 toSAO2 exhibits “1”, the majority decision element JD decides that thedata written to the memory cell pairs MCP are “1” and outputs “0” as thedecision result.

Therefore, by obtaining the majority logic of the data SAO0 to SAO2 anddeciding the data of three memory cell pairs MCP where the same data isto be written, even if a failure occurs in one of the three memory cellpairs MCP leading to abnormal data writing or abnormal data reading, itis possible to accurately decide whether the data written in the threememory cell pairs MCP is “1” or “0”. This allows an accurate reading ofthe data written in the memory cell array 6, thereby improving thereliability of the present semiconductor memory device.

Respective differential sense amplifiers SA0 to SA2 output the inversionsignal of the data written to their corresponding memory cell pair MCP.It follows that the data PD output by the majority decision element JDexhibits the majority logic of the data written to the three memory cellpairs MCP that are electrically connected to respective differentialsense amplifiers SA0 to SA2. Therefore, by latching the data PD as theyare in the latch circuit 10 and outputting the latched data as the dataread from the memory cell array 6, information written to the presentsemiconductor memory device can be accurately conveyed to a CPU and thelike.

The present preferred embodiment introduced a configuration whereinrespective differential sense amplifiers SA0 to SA2 invert data fromtheir corresponding memory cell pair. MCP for outputting to the majoritydecision element JD, which in turn outputs an inversion signal of themajority logic of the received outputs from the amplifiers SA0 to SA2.However, another configuration may also be possible wherein respectivedifferential sense amplifiers SA0 to SA2 output data of thecorresponding memory cell pair MCP as they are, then the majoritydecision element JD outputs the majority logic of the outputs receivedfrom the amplifiers SA0 to SA2 as it is.

Next, a detailed description is given of the structure of the start-updetector 12. FIG. 7 is a block diagram illustrating the structure of thestart-up detector 12. As shown in FIG. 7, the start-up detector 12according to the present preferred embodiment comprises a referencevoltage generation circuit 12 a for generating and outputting areference voltage for comparison Vref based on an external voltage Vexsupplied from the outside of the present semiconductor memory device.The start-up detector 12 further comprises a first voltage detector 12b, a second voltage detector 12 c and an AND circuit 12 d.

The first voltage detector 12 b compares the supply voltage Vdd and thefirst reference voltage Vref1 (not shown), and outputs the comparisonresult as a signal DET1. The second voltage detector 12 c compares theinternally boosted voltage BOOST and a second reference voltage Vref2(not shown), and outputs the comparison result as a signal DET2. The ANDcircuit 12 d carries out a logical product operation of the signals DET1and DET2 for outputting the result to the reading controller 2 as astart signal START.

When an end signal END output from the reading controller 2 becomes “1”,the first voltage detector 12 b terminates the monitoring of the supplyvoltage Vdd, thereby terminating the comparison of the supply voltageVdd and the first reference voltage Vref1. Similarly, when the endsignal END becomes “1”, the second reference voltage 12 c terminates themonitoring of the internally boosted voltage BOOST, thereby terminatingthe comparison of the internally boosted voltage BOOST and the secondreference voltage Vref2.

FIG. 8 is a circuit diagram illustrating the structure of the firstvoltage detector 12 b. As shown in FIG. 8, the first voltage detector 12b comprises resistors R1 and R2, PMOS transistors 120 to 122, and NMOStransistors 123 to 125. The PMOS transistors 120 to 122 and NMOStransistors 123 to 125 form a current-mirror differential amplifier. Theinternally boosted voltage BOOST is applied to a source of the PMOStransistor 120. A drain of the PMOS transistor 120 is connected tosources of the PMOS transistors 121 and 122. Further, a gate of the PMOStransistor 120 is supplied with the end signal END.

Gates of the PMOS transistors 121 and 122 are connected to each other,then further connected to a drain of the PMOS transistor 121. A drain ofthe NMOS transistor 123 is connected to a drain of the PMOS transistor121. A drain of the NMOS transistor 124 is connected to a drain of thePMOS transistor 122. A drain of the NMOS transistor 125 is connected tosources of the NMOS transistors 123 and 124. A source of the NMOStransistor 125 is supplied with a ground potential.

One end of the resistor R1 is supplied with the supply voltage Vdd, andthe other end of the resistor R1 is connected to one end of the resistorR2. The other end of the resistor R2 is supplied with a groundpotential, and a junction point O between the resistors R1 and R2 isconnected to a gate of the NMOS transistor 123. Gates of the NMOStransistors 124 and 125 are supplied with the reference voltage forcomparison Vref. The potential of a junction point P between the drainof the PMOS transistor 122 and the drain of the NMOS transistor 124 isoutput as a signal DET1.

It is noted that the reference voltage generation circuit 12 a has beensupplied with the external voltage Vex without fail before the supply ofthe supply voltage Vdd and the internally boosted voltage BOOST begins.Therefore, the reference voltage for comparison Vref has reached acertain value when the supply voltage Vdd and the internally boostedvoltage BOOST start to be supplied.

In the first voltage detector 12 b with the above-describedconfiguration, the potential at the junction point O between theresistors R1 and R2 is compared with the reference voltage Vref. Whenthe potential at the junction point O is larger than the referencevoltage Vref, the potential at the junction point P between the drain ofthe PMOS transistor 122 and the drain of the NMOS transistor 124approximates the internally boosted voltage BOOST, thereby rendering thesignal DET1 at a High level. On the other hand, when the potential atthe junction point O is smaller than the reference voltage Vref, thepotential at the junction point P approximates the ground potential,thereby rendering the signal DET1 at a Low level.

In the present preferred embodiment, for example, the supply voltage Vddis set at 1.8 V, and the reference voltage Vref is set at 1.2 V. Furtherin the present preferred embodiment, for example, the resistors R1 andR2 are set at 15 kΩ and 85 kΩ, respectively. Therefore, in a state whenthe supply voltage Vdd is fully ready for operation and steady, thepotential of the junction point O exhibits 1.53 V(=1.8×85 k/(15 K+85K)).

Therefore, the first voltage detector 12 b compares the value obtainedby multiplying the supply voltage Vdd by 85/100 with 1.2 V. Then, itfollows that the first voltage detector 12 b compares the supply voltageVdd with the value obtained by multiplying 1.2 V by 100/85, or about 1.4V. This value of 1.4 V corresponds to the above-described firstreference voltage Vref1.

As described above, the first voltage detector 12 b according to thepresent preferred embodiment does not compare the supply voltage Vdddirectly with the first reference voltage Vref1. Instead, the firstvoltage detector 12 b compares a voltage value (potential at thejunction point O) obtained by multiplying the supply voltage Vdd by acertain reduction rate with a voltage value (reference voltage Vref)obtained by multiplying the first reference voltage Vref1 by the samereduction rate, thereby to indirectly compare the supply voltage Vddwith the first reference voltage Vref1. As a result, when the supplyvoltage Vdd exceeds the first reference voltage Vref1, the signal DET1enters a High level. In contrast, when the supply voltage Vdd dropslower than the first reference voltage Vref1, the signal DET1 enters aLow level. Further, when the end signal END enters a High level, theinternally boosted voltage BOOST which serves as a supply voltage forthe first voltage detector 12 b no longer is supplied to thecurrent-mirror differential amplifier. This terminates the monitoring ofthe supply voltage Vdd in the first voltage detector 12 b.

FIG. 9 is a circuit diagram illustrating the structure of the secondvoltage detector 12 c. Modifications made to the first voltage detector12 b are that in the second voltage detector 12 c, resistors R3 and R4are provided instead of the resistors R1 and R2, and one end of theresistor R3 is supplied with the internally boosted voltage BOOST.

One end of the resistor R3 is supplied with the internally boostedvoltage BOOST, and the other end of the resistor R3 and one end of theresistor R4 are connected to each other. A ground potential is appliedto the other end of the resistor R4, and a junction point Q between theresistors R3 and R4 is connected to the gate of the NMOS transistor 123.The potential of the junction point P between the drain of the PMOStransistor 122 and the drain of the NMOS transistor 124 is output as asignal DET2. The rest is the same structure as that of the first voltagedetector 12 b, and the description thereof is omitted.

In the second voltage detector 12 c having the above configuration, thepotential of the junction point Q is compared with the reference voltageVref. Further, similar to the case of the first voltage detector 12 b,when the potential of the junction point Q exceeds the reference voltageVref, the signal DET2 enters a High level. In contrast, when thepotential of the junction point Q drops lower than the reference voltageVref, the signal DET2 enters a Low level.

In the present preferred embodiment, for example, the internally boostedvoltage BOOST is set at 6 V, and the resistors R3 and R4 are set at 70kΩ and 30 kΩ, respectively. Therefore, in a state when the supplyvoltage Vdd is fully ready for operation and the internally boostedvoltage BOOST is steady, the potential of the junction point Q exhibits1.8 V(=6.0×30 k/(30 k+70 k)).

Therefore, the second voltage detector 12 c compares the value obtainedby multiplying the internally boosted voltage BOOST by 30/100 with 1.2V. Then, it follows that the second voltage detector 12 c compares theinternally boosted voltage BOOST with the value obtained by multiplying1.2 V by 100/30, or 4 V. This value of 4 V corresponds to theabove-described second reference voltage Vref2.

As stated above, similar to the case of the first voltage detector 12 b,the second voltage detector 12 c according to the present preferredembodiment does not compare the internally boosted voltage BOOSTdirectly with the second reference voltage Vref2. Instead, the secondvoltage detector 12 c compares a voltage value (potential at thejunction point Q) obtained by multiplying the internally boosted voltageBOOST by a certain reduction rate with a voltage value (referencevoltage Vref) obtained by multiplying the second reference voltage Vref2by the same reduction rate, thereby to indirectly compare the internallyboosted voltage BOOST with the second reference voltage Vref2. As aresult, when the internally boosted voltage BOOST exceeds the secondreference voltage Vref2, the signal DET2 enters a High level. Incontrast, when the internally boosted voltage BOOST drops lower than thesecond reference voltage Vref2, the signal DET2 enters a Low level.Further, similar to the case of the first voltage detector 12 b, whenthe end signal END enters a High level, the internally boosted voltageBOOST which serves as a supply voltage for the second voltage detector12 c no longer is supplied to the current-mirror differential amplifier.This terminates the monitoring of the internally boosted voltage BOOSTin the second voltage detector 12 c.

FIG. 10 shows the voltage waveforms of the supply voltage Vdd and theinternally boosted voltage BOOST at power-on. Waveforms 130 and 131 inFIG. 10 represent the voltage waveforms of the internally boostedvoltage BOOST and the supply voltage Vdd, respectively. Further in FIG.10, “V1” represents the voltage value of the supply voltage Vdd in asteady state, and “V2” represents the voltage value of the internallyboosted voltage BOOST in a steady state. Furthermore, “timing t1”represents a timing at which a detection is made that the supply voltageVdd exceeds the first reference voltage Vref1, and “timing t2”represents a timing at which a detection is made that the internallyboosted voltage BOOST exceeds the second reference voltage Vref2.

As can be seen from the voltage waveforms shown in FIG. 10, in thepresent preferred embodiment, the supply voltage Vdd and the internallyboosted voltage BOOST are compared with a reference voltageindependently provided for respective voltages Vdd and BOOST.

Based on the above description, when the supply voltage Vdd and theinternally boosted voltage BOOST both exceed their respective referencevoltages to be compared with, the start signal START serving as anoutput signal of the AND circuit 12 d enters a High level. Further, whenat least one of the supply voltage Vdd and the internally boostedvoltage BOOST drops lower than their respective reference voltages to becompared with, the start signal START enters a Low level. When the startsignal START changes from a Low level into a High level, the readingcontroller 2 starts reading data from the memory cell array 6. Further,when the start signal START changes from a High level into a Low levelduring reading, the reading controller 2 again performs data readingfrom the memory cell array 6. Next, data reading operation from thememory cell array 6 is described in detail.

When power is turned on and the start signal START changes from a Lowlevel to a High level, i.e. a detection is made that the supply voltageVdd and the internally boosted voltage BOOST both exceed theirrespective reference voltages in the start-up detector 12, the readingcontroller 2 starts the operations of the internally provided addresscounter 2 a. This prompts the reading controller 2 to output the addresssignal ADR corresponding to either one of m number of word lines WL, andfurther to set the read control signal RDC to “1”.

The address decoder 3 receives and decodes the address signal ADR, thenoutputs the word line activation signal for reading RAWL. At this time,since the write control signal WRC is set to “0” by the writingcontroller 1, the gate signal switching circuit 13 outputs the word lineactivation signal for reading RAWL, which in turn is supplied to theword line WL. This prompts the activation of either one of m number ofword lines WL which corresponds to the value of the address signal ADR.Then the data of the memory cell pairs MCP connected to the activatedword line WL are ready for reading.

The voltage value to be supplied to activate the word line WL, or thegate voltage for reading Vg1 to be supplied to the control gates of thetrue-side memory cells MC and the bar-side memory cells MC of the memorycell pairs MCP to be read, is 5 V, for example. This 5 V is generatedfrom the internally boosted voltage BOOST. That is, the internallyboosted voltage BOOST is a preceding voltage in relation to the gatevoltage for reading Vg1.

The semiconductor memory device according to the present preferredembodiment comprises a voltage generation circuit (not shown) forgenerating 5 V from the internally boosted voltage BOOST for outputting.The voltage output from the voltage generation circuit is supplied as asupply voltage for the address decoder 3. Therefore, the voltage valueof the part of the word line activation signal for reading RAWL foractivating the word line WL output by the address decoder 3 is 5 V.Accordingly, 5 V is applied to the control gates of the true-side memorycells MC and the bar-side memory cells MC of the memory cell pairs MCPto be read.

As stated above, when one of the word lines WL is activated, the readingcontroller 2 controls the operations of the amplification circuit 8.This causes the output difference between the true-side memory cell MCand the bar-side memory cell MC included in respective memory cell pairsMCP connected to the activated word line WL to be amplified by one ofthe differential sense amplifiers SA0 to SA2. The amplified results arethen output to the respective majority decision elements JD. Each of themajority decision elements JD obtains the majority logic based on theoutputs from the differential sense amplifiers SA0 to SA2 and decidesthe data in the corresponding memory cell pairs MCP for outputting. Thedata PD output from each of the majority decision elements JD arelatched in the latch circuit 10 for outputting to a CPU and the like asthe read data RD0 to RDx−1.

Next, the reading controller 2 increases the count value of the addresscounter 2 a by one, thereby outputting the address signal ADR with avalue different from that of the previously output signal. The addresssignal ADR is decoded by the address decoder 3 and provided to the wordline WL. As a result, the word line WL different from the previouslyactivated word line WL is activated, and data are read from the memorycell pairs MCP connected to the newly activated word line WL. Finally,the read data RDO to RDx-1 are output from the latch circuit 10.

By repeating the above-described procedure, the reading controller 2reads data from all the memory cell pairs MCP of the memory cell array6. When the address counter 2 a reaches the count value corresponding toan address value assigned to the last word line WL to be activated, thereading controller 2 outputs the end signal END with the data “1” to thestart-up detector 12. This terminates the reading from the memory cellarray 6. Upon receiving the end signal END with the data “1”, thestart-up detector 12 stops monitoring the supply voltage Vdd and theinternally boosted voltage BOOST.

When the start signal START changes from a High level to a Low levelwhile data is read from the memory cell array 6, i.e. a detection ismade that at least one of the supply voltage Vdd and the internallyboosted voltage BOOST drops lower than their respective referencevoltages to be compared with while reading data, the reading controller2 resets the counter value of the address counter 2 a and stops itsoperations. Afterwards, when the start signal START changes to a Highlevel and a detection is made that the supply voltage Vdd and theinternally boosted voltage BOOST both exceed their respective referencevoltages, the reading controller 2 resumes the operations of the addresscounter 2 a. This causes the memory cell array 6 to be read again.

In this manner, when one of the supply voltage Vdd and the internallyboosted voltage BOOST drops lower than their respective referencevoltages, the reading controller 2 reads the memory cell array 6 again.Therefore, even if one of the supply voltage Vdd and the internallyboosted voltage BOOST drops temporally while the memory cell array 6 isread, leading to a failed data reading, the data reading from the memorycell array 6 can be performed securely.

Next, a detailed description is given of the writing operationsregarding the memory cell array 6 of the semiconductor memory deviceaccording to the present preferred embodiment. Upon receiving the writecommand WRCOM from an externally provided CPU and the like, the writingcontroller 1 sets the write control signal WRC to “1” and puts the NMOStransistors 5 a of the gate circuit for writing 5 in an ON state. Atthis time, the read control signal RDC is “0”, therefore the transistors7 a of the gate circuit for reading 7 a are in an OFF state.

The writing controller 1 outputs n bits of bit line selection signals D0to Dn−1 and applies the drain voltage for writing VPD to the bit line BLconnected to the memory cell MC whose threshold voltage is to beincreased, thereby activating the bit line BL. It is noted that thedrain voltage for writing VPD is generated from the internally boostedvoltage BOOST by a voltage generation circuit (not shown) provided inthe present semiconductor memory device. For example, the voltage valueof the drain voltage for writing VPD is 4.5 V.

Next, the writing controller 1 outputs the word line activation signalfor writing WAWL. Since the write control signal WRC is “1”, the gatesignal switching circuit 13 outputs the word line activation signal forwriting WAWL to the word line WL. Therefore, the word line WL connectedto the memory cell MC whose threshold voltage is to be increased isactivated, and the gate voltage for writing Vg2 is applied to thecontrol gate of the memory cell MC. As a result, hot electrons areinjected into the floating gate of the memory cell MC which is connectedto the activated bit line BL and activated word line WL, therebyincreasing the threshold voltage of the memory cell MC.

The writing controller 1 repeats the above-described operations,carrying out hot electron injections into the memory cell MC to changeits threshold voltage, thereby writing data to all the memory cell pairsMCP of the memory cell array 6.

The writing controller 1 according to the present preferred embodimentcan set the threshold voltage of each memory cell MC to four levels, forexample, 1.5 V, 3 V, 4.5 V and 6 V. In this example, the memory cell MChas the threshold voltage of 1.5 V when electrons are not injected intothe floating gate of the memory cell MC. The writing controller 1supplies a pulsed word line activation signals for writing WAWL aplurality of times to the word line WL connected to the memory cell MCwhose threshold voltage is to be increased. Therefore, a pulsed gatevoltage is given a plurality of times to the control gate of the memorycell MC whose threshold voltage is to be increased. The writingcontroller 1 decides the threshold voltage level of the memory cell MCbased on the number of times the pulsed gate voltage is applied to thememory cell MC. That is, as the value of the threshold voltage to be setfor the memory cell MC increases, the number of times the pulsed gatevoltage is applied to the control gate of the memory cell MC increases.

As described above, since the writing controller 1 can set the thresholdvoltage of each memory cell MC to four levels, rewriting data of thememory cell pair MCP is easily carried out during a wafer test, forexample. Detailed description of data rewriting is given in thefollowing. In the following description, it is noted that setting thethreshold voltage of the memory cell MC to 1.5 V, 3 V, 4.5 V and 6 V, isreferred to as setting the memory cell MC to “11”, “10”, “01” and “00”,respectively.

FIG. 11 is a flow chart illustrating operations of the semiconductormemory device of the present invention when the data of the memory cellpair MCP is rewritten during a wafer test. FIGS. 12A, 12B, 13A, 13B,14A, 14B, 15A and 15B show the values (threshold voltage values) set forthe memory cell MC in the steps of S1 to S4. FIGS. 12A, 13A, 14A and 15Ashow the true-side memory cell MC, and FIGS. 12B, 13B, 14B and 15B showthe bar-side memory cell MC.

As shown in FIG. 11, in the step s1, the data written to the memory cellarray 6 is erased using ultraviolet light. Then the values of thetrue-side memory cell MC and the bar-side memory cell MC are both set to“11”, as shown in FIGS. 12A and 12B. In the step s2, as shown in FIGS.13A and 13B, electrons are injected only into the floating gate of thebar-side memory cell MC, thereby setting the bar-side memory cell MC to“01”. This causes the true-side memory cell MC and the bar-side memorycell MC to exhibit the threshold voltage of 1.5 V and 4.5 V,respectively. As a result, the true-side memory cell MC has a smallerthreshold voltage than that of the bar-side memory cell MC and data “1”is written to the memory cell pair MCP.

It is noted that in the step s2, the writing controller 1 sets thethreshold voltage of the bar-side memory cell MC to one of the settablevoltages, but excluding the one with the largest value.

Next, in the step s3, electrons are injected only into the floating gateof the true-side memory cell MC, thereby setting the true-side memorycell MC to “00” (see FIGS. 14A and 14B). This causes the true-sidememory cell MC and the bar-side memory cell MC to exhibit the thresholdvoltage of 6 V and 4.5 V, respectively. As a result, the true-sidememory cell MC has a threshold voltage larger than that of the bar-sidememory cell MC, and the data of the memory cell pair MCP is rewrittenfrom “1” to “0”. In the step s4, after the wafer test is over, thevalues (threshold voltage values) of the true-side memory cell MC andthe bar-side memory cell MC are reset (see FIGS. 15A and 15B). Thisallows the difference between two input signals supplied to thedifferential sense amplifiers SA0 to SA2 to be secured to a satisfactorydegree, thereby preventing the malfunction of the semiconductor memorydevice of the present invention after being sent to the market.

FIG. 16 is a flow chart illustrating operations of the semiconductormemory device of the present invention when the threshold voltagesetting for each memory cell MC by the writing controller 1 is limitedto two kinds only. The example shown in FIG. 16 is a case where thethreshold voltage can be set to 1.5 V and 6 V. In this case, setting thethreshold voltage of the memory cell MC to 1.5 V and 6 V, is referred toas setting the memory cell MC to “1” and “0”, respectively.

As shown in FIG. 16, in step s11, the data written to the memory cellarray 6 is erased using ultraviolet light. Then the values of thetrue-side memory cell MC and the bar-side memory cell MC are both set to“1”. In step s12, electrons are injected only into the floating gate ofthe bar-side memory cell MC, thereby setting the bar-side memory cell MCto “0”. This causes the true-side memory cell MC and the bar-side memorycell MC to exhibit the threshold voltage of 1.5 V and 6 V, respectively.As a result, the true-side memory cell MC has a smaller thresholdvoltage than that of the bar-side memory cell MC and data “1” is writtento the memory cell pair MCP.

In this example, when the data of the memory cell pair MCP is rewrittenfrom “1” to “0”, i.e. when the threshold voltage of the true-side memorycell MC is changed to be larger than that of the bar-side memory cellMC, it is necessary to erase all the data from the memory cell array 6by applying ultraviolet light in step s13. This step is necessary forthe increased threshold voltage of the bar-side memory cell MC to bereset since the threshold voltage setting for the memory cell MC islimited to two kinds only.

After the data of the memory cell array 6 is erased in the step s13, thevalues of the true-side memory cell MC and the bar-side memory cell MCare all set to “1”. In step s14, electrons are injected only into thefloating gate of the true-side memory cell MC, thereby setting thetrue-side memory cell MC to “0”. This causes the true-side memory cellMC and the bar-side memory cell MC to exhibit the threshold voltage of 6V and 1.5 V, respectively. As a result, the true-side memory cell MC hasa threshold voltage larger than that of the bar-side memory cell MC andthe data of the memory cell pair MCP is rewritten from “1” to “0”.

As described above, when the threshold voltage setting for the memorycell MC is limited to two kinds only, it is necessary to erase data onceupon rewriting the data of the memory cell pair MCP. However, if thethreshold voltage can be set to four levels, as in the present preferredembodiment, it is possible to rewrite data of the memory cell pair MCPwithout erasing the data. This enables the data rewriting to beperformed easily.

The present preferred embodiment described a case when the thresholdvoltage can be set to four levels. However, the same advantageous effectcan be achieved even if the threshold voltage setting for the memorycell MC is limited to three kinds only. In other words, if the thresholdvoltage can be set to three or more levels, data can be rewritteneasily.

When the threshold voltage can be set to three levels of 1.5 V, 3 V and4.5 V, for example, the threshold voltage of the true-side memory cellMC is set to a level lower than that of the threshold voltage of thebar-side memory cell MC, while setting the threshold voltage of thebar-side memory cell MC to a level except the largest voltage. That is,in the present example, the threshold voltages of the true-side memorycell MC and the bar-side memory cell MC are set to 1.5 V and 3 V,respectively, thereby writing the data “1” into the memory cell pairMCP. Upon rewriting the data of the memory cell pair MCP from “1” to“0”, the threshold voltage of the true-side memory cell MC only ischanged, thereby setting the threshold voltages of the true-side memorycell MC and the bar-side memory cell MC to 4.5 V and 3 V, respectively.This allows the data of the memory cell pair MCP to be rewritten withoutinvolving a data erasing operation.

As described above, in the semiconductor memory device according to thepresent preferred embodiment, the writing controller 1 can individuallyset the memory information for each of the memory cells MC of the memorycell array 6. Under this configuration, upon writing data to the memorycell array 6, it is possible to set a large difference in the memoryinformation between two memory cells MC included in one memory cell pairMCP. This prevents a malfunction of a circuit for detecting a differencein such memory information, such as the differential sense amplifiersSA0 to SA2 according to the present preferred embodiment. As a result,the semiconductor memory device of the present invention will have animproved reliability.

Further, in the present preferred embodiment, the internally boostedvoltage BOOST obtained by boosting the supply voltage Vdd is used as asupply voltage for the first voltage detector 12 b. Accordingly, even ifthe supply voltage Vdd drops to a certain extent, a supply voltagecapable of operating the first voltage detector 12 b can be secured.This can suppress the malfunction of the first voltage detector 12 b.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

1. A method of rewriting data of a nonvolatile semiconductor memorydevice, said nonvolatile semiconductor memory device comprising a memorycell array having a plurality of memory cells, and a writing controllerfor writing data to said memory cell array, wherein said memory cellarray stores data in a nonvolatile state with respect to each memorycell pair composed of two memory cells, the data storing being executedbased on a difference in memory information between said two memorycells, said writing controller is capable of setting a threshold voltageof each of said memory cells of said memory cell array to three or morelevels, said writing controller sets said memory information of each ofsaid memory cells individually by setting the threshold voltage of eachof said memory cells of said memory cell array, and said method ofrewriting data comprises the steps of: (a) writing data to said memorycell pair; and (b) rewriting said data written in said step (a), whereinin step (a), the threshold voltage of one memory cell included in saidmemory cell pair is set to a lower level than that of the thresholdvoltage of the other memory cell in said memory cell pair, while saidthreshold voltage of said other memory cell is set to a level chosenfrom said three or more levels except the greatest one, and in step (b),the threshold voltage of said one memory cell is increased to a levelgreater than the threshold voltage of said other memory cell.